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Geologic Time and Absolute Dating

Geologic Time and Absolute Dating

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Geologic Time and Absolute Dating

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  1. Geologic Time and Absolute Dating

  2. Review: Atomic Structure • Atom • Basic unit of an element • Composed of protons and neutrons (nucleus) surrounded by electrons • The identity of an atom is determined by the number of protons the atom has

  3. Example: Krypton • Krypton’s atomic number is 36, therefore • A neutral krypton atom has 36 protons and 36 electrons • If atomic number ≠ 36 ≠ krypton • The number of neutrons =atomic number-atomic weight (rounded up)=48 • The number of neutrons can vary without altering the identity of an atom—Isotopes

  4. Isotopes • Isotopes are like people—some are stable; some are not • And, like people, it’s the unstable ones that attract our attention the most Stable… Seriously unstable…

  5. Radioactivity • Unstable isotopes are radioactive—their nuclei will decay over time • A any radioactive isotope is called a “parent” isotope • The decay product is called the “daughter” isotope • When an isotope decays, they do so in one of three ways…

  6. Alpha emission • Nucleus emits two protons and two neutrons • Plutonium-240 decays to uranium-236 • Beta emission • Nucleus emits an electron • Radium-228 decays to Actinium-228 • Electron capture • An atom’s nucleus captures an electron which reacts with a proton creating a neutron • Carbon-11 decays to Boron-11 • In a nutshell: When the nucleus decays, a new, more stable isotope is created Electron capture

  7. Radioactive Decay and Popcorn…yummy • Radioactive decay is a spontaneous and irreversible process • Ex. popcorn

  8. If the Decay of an Atom Occurs Randomly, How is it Useful to Us? Sample of actinium Even a small sample is composed of billions of actinium atoms (Ac-227) After 22 years, exactly half of the Atoms have decayed to thorium-227 All isotopes of actinium are unstable and will decay over time. Since every atom has a certain probability of decaying, on average, half of the atoms in a given sample will decay to a (more) stable daughter isotope over a set period of time Actinium-227 has a half-life of 22 years

  9. Half-lives • We can use the half-life of an isotope to figure out the age of a rock • How can we do this? • Half-lives are constant • Actinium-227 always has a half-life of 22 years • As the parent decays the daughter accumulates • Older samples = higher number of daughter isotopes

  10. Example 100 grams of Bob-12 • If we have a rock with 100 grams of a particular isotope (Bob-12) • Bob-12 decays to Joe-11 and has a half-life of 400 Million years • How old is our sample if only 25 grams of Bob-12 remain? • Our sample is 800 My old 400 My (one half-life) 50 grams Bob-12 50 grams Joe-11 + = 100 g 400 My years (now two half-lives have passed) 25 grams Bob-12 75 grams Joe-11 + = 100 g

  11. Good vs. Bad • Isotopes with long half-lives are good for old rocks • Young materials are best dated by short lived isotopes

  12. Commonly Used Isotopes